A 52-year-old Vietnamese man who ate chicken that had died of a disease has been infected with the H5N1 bird flu virus, the country´s first human case in a month, a health official said on Monday.

The man from the northern province of Vinh Phuc was hospitalised with a fever last Thursday after eating the chicken with his family, a provincial medical official told Reuters by telephone.

"We have taken blood and saliva samples of four family members for bird flu tests and those of the man were confirmed positive," he said.

Three family members are under surveillance and the area surrounding their home in Vinh Phuc, 63 km (39 miles) northest of Hanoi, was disinfected, the official said.

Vietnam´s last human H5N1 case was in early April, and no new outbreaks among poultry have been reported for nearly a month.

The virus has killed 52 people in Asia since 2003, 36 in Vietnam, 12 in Thailand and four in Cambodia.

Vietnamese doctors treating a 20-year-old Cambodian woman said on Monday her condition was worsening since she was transferred to a Vietnamese hospital from Cambodia´s Kampot province last Wednesday. She has been on a respirator, they said.

A researcher at the Ho Chi Minh City-based Pasteur Institute said preliminary tests on the Cambodian woman showed she was not infected by the virus, but more tests were being conducted.

The woman is from the same area where four Cambodians died earlier this year from bird flu.

Health experts fear the virus could mutate into a form which can pass easily among humans and trigger a global pandemic that could kill millions.